Interference-schlieren apparatus



Dec. l, 1953 T. W. ZOBEL INTERFERENCE-SCHLIEREN APPARATUS Filed July 25,1950 5 Sheets-Sheet l IN V EN TOR. 77-/5000/9 I4( OEL BY 0m Dec. 1, 1953Filed July 25, 1950 T. W. ZOBEL INTERFERENCE-SCHLIEREN APPARATUS '3sheets-sheet 2 INVENTOR.

De 1, 1953 T. w. zoBEL 2,660,915

INTERFERENCE -SCHLIEREN APPARATUS Filed July 25, 195o s sheets-sheet sINVENTOR.

Patented Dec. l, i953 UNITED STATES PATENT y(IFFICE 3 Claims.

sec. 266

(Granted under Title 35, )U. S. Code (1952),

1 The invention described herein may be manufaotured and used by or forthe Government for governmental purposes without payment to me of anyroyalty thereon.

This invention relates to optical arrangements and methods for examiningthe density of materials and test mediums in a qualitative andquantitative respect such asdisclosed in my U. S. Patent No. 2,256,855iiled'? November 1939 and patented on 23 September 1941.

It is expedient in that patent to examine the medium to be tested withtwo light rays traveling in the same direction. Efundamentally it ispossible with the Schlieren light ray, to exam ine the medium to betested in a ray traveling in the opposite direction, or in exceptionalcase, at some other angle. In all cases, supplementary to knownadvantages, Where the Schlieren light ray penetrates the medium to betested in another direction from the interference ray, a separate lightsource is normally necessary. The Schlieren ray must be superimposed onthe interference ray by means of a partially coated medium and in thesame manner separated from it. This process increases the light lossconsiderably, and other disadvantages relative to the nltering outprocess of the superimposed Schlieren light ray are present.

In the subject invention it is possible to use a single light source,dispense with filters and filtering out processes, the interferencelight rayy being used in the interference fringe method and ink theSchlieren method. By this method the light ray travels divided through afour plate system unfiltered, but before one of the divided light raysreaches the fourth plate, a portion of the ray is rellected or branchedoff for use in the Schlieren method.

In the former arrangements in my patent,`

above referred to, the distance from the medium ll to be tested throughthe coated (partially silvered) reilector plate It to the screen I6 isnot the same as the distance through the interference plate l2, makingit necessary to provide lenses or concave mirrors having different focallengths when the object under investigation is to be projected on thescreen shown in the patent is to be the same relative size for bothpictures.

According to the present invention, an inherent simplication of theprocesses is reached because both light ray paths are at the sameoptical distance from the object of the test, so that where comparativepictures of interference fringes and Schlieren .images are projected in2 side-bynside relation on the image screen they are of the samerelative size making it necessary to use only one lens, or concavemirror for both light beams with the Schlieren knife edge lo'` cated inthe focal plane of that lens or concave mirror. Both light beams may beinclined rel ative to each other to produce two separate lightconcentrations in the focal plane of lens Id with the Schlieren knifeengaging only the Schlieren beam.

Another example from the physics of air new may be noted: With thenowing of an air stream around a model in a test medium, such as theWings of an aircraft in the subsonic and supersonic ranges, it isfrequently necessary to install the Schlieren knife edge with the edgeapproximately parallel to the axis of the model, making boundary layersand shock Waves on the image screen bright at the upper half of themodel, and Ydark on the opposite or lower side of the' model, blendingtogether at the contour of the image of the model. The image is,however, falsied on one side and the true pronle section of the modelcan only be recognized on one side;

In the present invention this disadvantage is avoided by an improveddark eld arrangement. Instead of a Schlieren knife edge I may utilize avery small opaque plate, inserting the same in the concentrated lightfrom the source in the focal plane, the small plate being supported by atransparent plate in order to eliminate any disturbances caused by theprocess of mounting, permitting the light ray to be refracted out in anydirection as soon as the density change takes place in the testedmedium. The direction of the density change now is not eiiected as inthe case with the Schlieren knife edge stop member which means that heatas well as coolness yields only brightness on the image screen. Thisadvantage makes the method appear'to cause the image on the image screenlift itself away as a darkened silhouette, from bright surroundings onthe screen.l

Other objects and advantages of the invention will become apparent fromthe following description, taken in connection with the accompanying.drawings in which like reference characters' refer to like parts in theseveral figures.

Figure l is a schematic view of an arrangerment incorporating myinvention and utilizing lenses;

Figure 2 is a modified arrangement utilizing the Z shapedarrangement ofthe light inlet and exit portions of the apparatus, utilizing concavereectors instead of lenses;

Figure 3 illustrates a further modification of my apparatus, utilizing asymmetrical arrangement for the concave reflectors and optical system;

Figure ll is a top view of the portion of the apparatus below the level4-4 looking in the direction of the arrows, parts shown in section;

Figure 5 is a vertical sectional view taken about on the plane indicatedby line 5-5 of Figure 3;

Figure 6 illustrates a further modified embodiment in which the lightentrance arrangement is similar to Figure 3, but the light exitarrangement is modied to eliminate the useof the solid lens means shownin Figures 3 and 4;

Figure 7 is a horizontal schematic view taken about on the line 'i-l ofFigure 6 looking in the direction of the arrows; and

Figure 8 is a vertical sectional view taken about on the line sof Figure6, looking in the direction of the arrows, and

Figure 9 is a view similar to Figure 8 in which the adjacent side of theSchlieren and interference beams are substantially parallel.

Reference numeral l in Figure 1 indicates a source of concentrated lightdisposed in the rear of a light slit or elongated light aperture i2 witha lens system 3 having a second light aperture 4 for the converging beamof light to pass therethrough to a collimating lens 5 for projecting acollimated beam of light ti onto a partially silvered beam splittingmirror l, silvered on the side facing the light source to pass aboutseventy percent of the light therethrough and reflect thirty percent.rlhe mirror l is preferably inclined across the collimated light beam tat an angle of 45 degrees and constitutes a light beam splitting means.

Two at opaque mirrors are inclined, one across the collimated beam t andthe other across reflected beam e, these mirrors being indicated inFigure 1 by the reference numerals lil and li constitute reiiectingmeans for reflecting the two partial light beams 6 and 3 toward eachother again. The reflected portion of the thirty percent collimatedlight beam 3 and the portion of the seventy percent light beam 9reflected by mirror lil extend in parallel paths.

A semi-transparent beam splitting plate l?. is inclined across both ofthe divided light beams 8 and il at their point of intersection, thissemitransparent plate having a coating capable of passing fifty per centof the beam 3 therethrough and reflecting fifty percent of the beam 9 inthe same direction as the axis of the beam 8 before the passage of thebeam 9 through the semitransparent reflector I2.

A full mirror I3 is inclined across the beam S after its passage throughthe beam combining reilector l2 reecting the beam 8 in almost but notquite parallel relation to the beam 9 after the beam 9 leaves the opaquemirror ill. The divided light beams S and S are thereby recombined andpass as a light interference beam through either a large condensing lensor two separated lenses i4 having a focal plane at l5, and are projectedon a recording or image screen i6. This arrangement just described formsa four-plate interferometer system somewhat similar to that disclosed inmy patent No. 2,256,855 (to Theodor Zobel), issued September 23, 1941.The test medium is interposed in the partial light beam e, as indicatedat ll. Some compensating means must be. provided in the;

partial beam t to compensate forr thetransparent walls of the testchamber containing the test medium.

A semi-transparent flat mirror lli is inclined across the partial lightbeam 9 between the test medium il and the semi-transparent mirror l2 forreiecting a portion of the test il laterally, for use in Schlierenmethod. This last reiiected or Schlieren beam, indicatcdat lil, strikesan opaque or full mirror 2S disposed in parallel relation to the mirrori8 reiiecting the Schlieren beam through the condensing lens te inadjacent almost parallel relation to the recombined beam; afterthereectionthereof by the opaque rnirror i3. l The condensing lens laconverges the Schlieren beam i9 and the recombined interference beamthrough two close together focal points in the focal plane l5, the twobeams then diver-ging from their focal points are projected onto thedual image receiving or recording screen it in close side-by-siderelation, providing two side-by-side images of the test medium ofsimilar proportions, one image being a Schlieren image and the otherbeing the interference The Schlieren knife edge orV stop member 2lidisposed to engage the converging Schlieren beam inthe focal plane it,provides the necessary means fordarkening of the image eld to producethe Schlieren image.

The above described system and apparatus provides an inherentsimplification of the appan ratus since both light pathsare'synchronized at the same distance from the test medium lll, bothinterference beams passing through the common iens means it or twoseparate close together lenses which converges them through two focalpoints l5 produces a pair of side-by-side images on the image screenAIt. Both pictures produced are identical in size, and juxtaposedrelative to each other, and thereforeA comparison can be easily andsatisfactorily made by observations, or a recordation is possible when asensitized surface is provided at the image screen.

The arrangement shown in Figure l has the advantage of producing clearcut optical representations and since the light source is in the opticalaxis and the geometries of the ray is symmetrical no distortion orastigmatism is possible, providing a lens of good optical quality isused and has suiicient diameter to include the two parallel interferencelight beams.

Referring to Figure 2, a-pairof concave optical reflectors or mirrors22, 22 are used, arranged to provide Z shape light inlet and lightoutlet means for the combined dual light ray interference system shownin this gure. These concave reflectors have their optical axes inclinedso as to have their axes oiiset relative to the main collimated beampassing through the optical arrangement and the final dual interferencebeams. The reference numeral 23 indicates a concentrated light sourcepreferably of monochromatic light having an optical slit or aperture 2tadjacent the light source 23 as shown in Figure 2. A conventional goodquality lensr 25 gathers the light passing through the light slitaperture 2G and concentrates the same through the light concentration 25with a light aperture 2l located at the focal point of the concavereiiector 22. The reflecting surface of the concave reflector preferablyis parabolic, indicatedat 28, reflecting collimated light beam4 29 ontothe adjacent refleeting surface'of a semi-transparent beam splittingmirrorv 3c, silvered toreiiect substantially 0%4 ofV thecollimated beamQuand. passing substantially '70%v therethrough,r forming respectivelytwo divergingV partial beams of collimated light indicated at 3| and 32.Full mirrors 33 and 3ft are inclined respectively across the-collimatedbeamsk 3| and 32 and are disposed in almost parallelrelation to the.beam splitting mirror Lit. The two opaque reectors33 and 34 reilect thetwo partial light beams 3l-32 across eachother in intersecting relationto form the respective beams 35 and 36, a semi-transparent beamsplitting plate 3l being inclined across the beams 35 and 35 at theirpointv of intersection, having a semi-silvered reflecting surface facingthe mirror 3e, for passing substantially 50% of the beam 35 therethroughand reecting substantially 50% of the beam 36 in coincident relation tothe beam 35, forming a bea n recombining mirror for combining the twoportions of the intersectingv collimated beams 3-3S- A test medium 36 tobe inspected is interposed'in the beam 36 to produce light interference'phenomena in the recorrlbinedv beam 3Q. v

A beam splitting plate or semi-transparent mirror di? is inclined acrossthe beam St which contains the test medium 3,5, having 50% reflectingsurface for reflecting substantially 59% of the test beam 36 laterallyso that the portion reaching the beam splitting mirror 3l issubstanrplane 4S onto an image screen ci.

. A lSchlieren knife edge or stop member se engages the convergingreflected Schlieren beam at its focal point et. Two side-by-sidepictures are produced on the common image screen itat the portionsindicated at 49 and 5b on the image screen. By utilizing a 30%-70%partial transparent reflecting surface on the beam splitting plate 3Sand 50% partial transparent reflectors as indicated at 3? and fili theillumination of the two interference beams reaching the screen ft' issubstantially equal. As mentioned before, the images are disposedside-by-side; have similar sizes, so that the comparison of the lightwave interference patterns formed thereby is easily and satisfactorilymade.

Figure 3 illustrates a slightly diierent arrangement, although thefour-plate inteferometer' arrangement as shown is substantially similarVto that shown in Figures 1 and 2. Means are provided in thismodification for utilizing a symmetrical optical system, providing ashort focus concave mirror instead of the solidv lens in Figure lor thelong focus mirror in Figure 2 so that the illumination can be materiallyincreased over that shown in Figure 2, and the light loss and otherobjections inherentv with largethick solid lenses are not present in thearrangement about to be described. The substitution of the concavemirrors having short focal lengths, particularly at the light entrance,`for the lenses employed in Figure l alords increased illumination andgreater covering power. In Figure 3A a concentrated source ofmonochromatic light is indicated at 5l, a light slit or aperture at 52,and a small lens 53 for concentrating the light passing through thelight aperture 52 on an inclined vsurround the concentrated beam and issupported by a transparent plate 55. The focal point of the beamstriking the small mirror plate 5c is located at the focal point of theconcave reflector 5t and. the beam is reected by the concave mirror 5tas a collimated beam 5l.

A beam Vsplitting partially transparent plate 53 is rinclined across thebeam said mirror 53 having a partially transparent 30% reflectingsurface for reflecting substantially of the beam 5l, indicated at 59,and passing substantially 70% of the beam 5'! therethrough, indicated atet, forming the two diverging partial light beams of collimated light,indicated at t9 and tti. Opaque reflector plates or full mirrors iii,'33 are inclined across the beams 5S and to and disposed parallel to thebeam' splitting plate 58, reflecting the two beams 5t and @il acrosseach other in intersecting relation., A semi-transparent beam splittingplate 53 is inclined across the reflectedl beam tu, reflectingsubstantially. of the beam El) laterally for use in forming theSchlieren beam ill. A semi-transparent beam splitting recoinbining plate5d is inclined `across the intersecting remaining portion of the beamand the beam tially 35% rather than 70%. Opaque iat mirrors reflectingthe two converging be fiected converging beam e@ 59 at their point ofintersection, producing the recombined interference beam t5.

A test medium es is interposed in the beam preferably between thekopaque mirror sa and semi-transparent beam splitting plate The y portionof the beam to which is reflected by the seini-transparent mirror isindicated this beam being again reflected by an opaque mirror 53. Asecond opaque reflector plate or mirror iii is inclined acrossthe-combined interference beam te for reflecting the same in adjacentparallel relation to the Schlieren beam this reflected beam beingindicated at il. A pair of concave reflectors with parabolic reflect' Afaces l2 and i3 are provided, positie-nei, in the respective beams 5&3and il for reilecting and converging the beams 65 and il on their'af/:es or an inclined semi-transparent rei= laterally out of theSchlieren and interior 7icc beams and through the respective focalpoints of the concave mirrors i2 and best seen in Figure fl. Atransparent supporting plate 'it positioned across the two converginginterference beams adjacent their respective roca-l points iii, theplate it having one small opaque .spot or cbstruction at the center ofthe fichlieren beam slightly smaller than the diameters ci the re toproduce the Schlieren picture on the image scree Lenses i8 and 'Haaredisposed in the tivo beams iii; 'li substantially` at the focal pointsand an aforesaid Schlieren knife or stop member may disposedv to engagethe edge of the Schlieren beam 69. spaced parallel rene-eter plates andSi, are dispose in the respective 69h and lib between the lenses and theimage plane or screen si?, the reflectors er and t5 Ycomprising meansfor locating the adjacent edges of the two pictures in juxtaposedrelation on the screen S5. This part of the arrangement is best shown inFigure 5.

In Figures 6, 7 and ci of the drawings a fourplate interferometer systemis shown which is somewhat similar tothe arrangement illustrated inFigure 3, except that the light exit arrange-l ment of the apparatus isdifferent. For this rea- Figure 3 to indicate similar or identicalparts,

with the exception that these reference numerals are each primed. Forinstance, the light source in Figure 3 is indicated at 5i and thecollimating reflector atiit, while in Figure 6 the similar light sourceis indicated at iii' and the collimating reflector at The light exitportion of the apparatus shown in Figure 5 is slightly modined,comprising a transparent supporting plate which is disposed across bothof the interference beams te and li', this plate being positionedslightly beyond the focal point ci the two concave reflecting surfacesl2 and i3. Small opaque reflectors and supported by the plate de areinclined across the converging yportions and l la ci the respectivebeams es' and il', just in oi' the focal points for reflecting theconverging beams laterally through their rea spective focal points. Thelittle mirror can be used simultaneously as the Schlieren lrnie edge toproduce Schlieren eiect on screen Another arrangement is possible by themirrors and 8B a little larger and moving the slightly closer to theconcave mirrors 'E3 and Then the knife edge di? is disposed to engageSchlieren beam its focal point te outside or the parallel beam Se.

-Figure 3 discloses the exit portion comprising a pair of inclinedparallel opaque reflectors 9i, 92 positioned in the beam Sila betweenthe focal point il@ of the screen Sil', together with a second pair ofparallel opaque reflector' plates B3 and Sel disposed in the beam 'l la,so as to project the Schlieren beam @so and the interference beam 'iidwith their axes parallel and their adjacent edges adjacent to each otherat the image viewing or recording screen Se.

Figure 9 discloses a slightly different arrange ment of the parallelopaque mirrors, providing means for positioning the adjacent sides ofthe two diverging interference beams 5ta and ila in juxtaposed parallelrelation substantially throughout their entire length. The inclinedparallel mirrors in the beam 69a are indicated at S5, @t and those inbeam 'lla are Sl', t3.

In the device as shown in all of the figures of the drawings a singlelight source is used, the beam is split or divided, as in the usualfour-plate interferometer arrangement, and then recoin bined, to formthe light interference beam containing fringes, one o the divided beams,prising the test beam passing through the testi -g medium while theother passes outside or" the test medium provides the variation in lightdensity between the two beams to produce the light Wave interferencephenomena. rIhe test beam is split to provide a third beam bciorereaching the semitransparent recombining plate for causing the Schlierenpicture. Both of these beams are then projected in parallel sidebysiderelation onto the single viewing or recording screen itt" or ii toproduce sideby-side Schlieren and interference fringe pictures. Byutilizing a reilecting surface having a 30% semitransparent coating onthe rst splitting pla-te t3, the splitting plate e3 for the Schlierenpicture divides the intensity of the illumination in the test beambefore it reaches the iecornbining plate till so that the illuminationof the Schlieren image and the illumination or the interference fringeimages are substantially equal.

Another advantage is the utilization ci a single light source forproducing both the Schlieren and the interference fringe images,together with means for keeping the images uniform in size, as Well asuniformly and equally illuminated.

What I claim is:

1. In an optical arrangement for the investigation of light densityfield of a test medium, nrst and second comparative test means forexploring the density elds, a single light source ior both of said testmeans having a light aperture, means for collimatng the light passingthrough the aperture into an enlarged collimated light beam, said iirsttest means including a Se per cent silvered flat mirror inclined acrossthe collimated light beam for reflecting approximately Se per cent ofthe light beam and. passing therethrough about 70 per cent of the lightbeam, to forni two diverging collimated partial light beams, a pair offiat opaque full mirrors, one inclined across each of the partial lightbeams to relect the diverging light beams toward each other, to form twospaced rectangular light paths or Substarin tially equal length, said'70 per cent partial light beam adapted to traverse a test medium, arecombining semi-transparent lat mirror inclined across the 3i) per centpartial light beam at the intersecting axes of the two partial lightbeams, said semi-transparent mirror having a semiransparent reilectingsurface for reflecting substantially 5i) per cent of the 'lo per centpartial light beam passing the test medium and striking the recombiningmirror a coincident direction to 5o per cent of the Si) per cent partiallight beam passing through the last mentioned somitransparent`reflecting surface to reunite the last mentioned two partial lightbeams to form a single light interference beam, a dat opaque mirrorinclined across the interference lig t beam for reflecting theinterference light beam laterally in a predetermined direction, saidsecond corn parative test means including a third mirror having asemi-transparent dat reilecting surace inclined across the path of thepartial light traversing the test medium and located between the testmedium and the semi-transparent recombining mirror for reflectingapproximately 5o per cent or the partial light traversing the testmedium in a lateral, almost but not quite, parallel direction divergingrelative to the re flected portion o the interference bean-i to form aSchlieren beam, `a iiat opaque full mirror inclined across the path or"the sai-d lateral fiected Schlieren beam for reflecting the Schlierenlight beam in almost but not quite parallel space-:i side-by-siderelation to the interference beam and in the same direction, singlefocusing ineens disposed in the path of both of the last mentionedreflected parallel light beams at equal optical distances from the testmedium for converging the beams through tivo juxtaposed points in thefocal plane of the single ocusing means to form a pair or side-bysideinterference Schlieren images of identical on the in ge plane or" thesingle Focusing means, and a "Heren stop member positioned to engage theS eren light beam in said focal plane to form a darkened iight eld forthe Schlieren iinage in sait' nage plane.

2. Apparatus as claimed in clamL l in which the focusing ineans forconverging the tivo partial light beams through two juxtaposed pointscomprises a single concave reiector having the two said points locatedon a plane passing through its focal point and includes an image screenlocated in the image plane ci the 'last mentioned concave reiector forreceiving the light images of the first and second comparative testmeans in juxtaposed side-byside relation.

3. Apparatus as claimed in claim l in which 75 said single focusingmeans for converging said (i il two partial light beams through said twojuxta- References Cited in the le of this patent posed points in itsfocal plane comprises a single UNITED STATES PATENTS lens recewing both0f said reflected lnterferenoe and Schlieren beams therethrough, and animage Numbel Name Date screen in the image plane of said single lens 51,938,992 Baker et al --Dec'12' 1933 for receiving the interference andSchlieren fgln-a-n- St 293 ima es thereon in side-b -side relation. f 1p g y 2,570,219 Drake oct. 9, 1951 THEODOR W. ZOBEL. FOREIGN PATENTS 10Number Country Date 720,333 Germany May 1, 1942

